diff --git a/android/Android.bp b/android/Android.bp
index f3a3850..6450a06 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -110,6 +110,7 @@
         "paths_test.go",
         "prebuilt_test.go",
         "rule_builder_test.go",
+        "sdk_test.go",
         "singleton_module_test.go",
         "soong_config_modules_test.go",
         "util_test.go",
diff --git a/android/sdk.go b/android/sdk.go
index 1518a87..1d63d7a 100644
--- a/android/sdk.go
+++ b/android/sdk.go
@@ -396,6 +396,25 @@
 func (r *sdkRegistry) copyAndAppend(registerable sdkRegisterable) *sdkRegistry {
 	oldList := r.list
 
+	// Make sure that list does not already contain the property. Uses a simple linear search instead
+	// of a binary search even though the list is sorted. That is because the number of items in the
+	// list is small and so not worth the overhead of a binary search.
+	found := false
+	newPropertyName := registerable.SdkPropertyName()
+	for _, r := range oldList {
+		if r.SdkPropertyName() == newPropertyName {
+			found = true
+			break
+		}
+	}
+	if found {
+		names := []string{}
+		for _, r := range oldList {
+			names = append(names, r.SdkPropertyName())
+		}
+		panic(fmt.Errorf("duplicate properties found, %q already exists in %q", newPropertyName, names))
+	}
+
 	// Copy the slice just in case this is being read while being modified, e.g. when testing.
 	list := make([]sdkRegisterable, 0, len(oldList)+1)
 	list = append(list, oldList...)
diff --git a/android/sdk_test.go b/android/sdk_test.go
new file mode 100644
index 0000000..51aeb31
--- /dev/null
+++ b/android/sdk_test.go
@@ -0,0 +1,53 @@
+// Copyright (C) 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package android
+
+import "testing"
+
+type testSdkRegisterable struct {
+	name string
+}
+
+func (t *testSdkRegisterable) SdkPropertyName() string {
+	return t.name
+}
+
+var _ sdkRegisterable = &testSdkRegisterable{}
+
+func TestSdkRegistry(t *testing.T) {
+	alpha := &testSdkRegisterable{"alpha"}
+	beta := &testSdkRegisterable{"beta"}
+	betaDup := &testSdkRegisterable{"beta"}
+
+	// Make sure that an empty registry is empty.
+	emptyRegistry := &sdkRegistry{}
+	AssertDeepEquals(t, "emptyRegistry should be empty", ([]sdkRegisterable)(nil), emptyRegistry.registeredObjects())
+
+	// Add beta to the empty registry to create another registry, check that it contains beta and make
+	// sure that it does not affect the creating registry.
+	registry1 := emptyRegistry.copyAndAppend(beta)
+	AssertDeepEquals(t, "emptyRegistry should still be empty", ([]sdkRegisterable)(nil), emptyRegistry.registeredObjects())
+	AssertDeepEquals(t, "registry1 should contain beta", []sdkRegisterable{beta}, registry1.registeredObjects())
+
+	// Add alpha to the registry containing beta to create another registry, check that it contains
+	// alpha,beta (in order) and make sure that it does not affect the creating registry.
+	registry2 := registry1.copyAndAppend(alpha)
+	AssertDeepEquals(t, "registry1 should still contain beta", []sdkRegisterable{beta}, registry1.registeredObjects())
+	AssertDeepEquals(t, "registry2 should contain alpha,beta", []sdkRegisterable{alpha, beta}, registry2.registeredObjects())
+
+	AssertPanicMessageContains(t, "duplicate beta should be detected", `"beta" already exists in ["alpha" "beta"]`, func() {
+		registry2.copyAndAppend(betaDup)
+	})
+}